1. Field of the Invention
The present invention relates generally to a method and an apparatus for adjusting the direction of an antenna in a system receiving satellite broadcasting waves directly from a satellite, and more particularly to a method and an apparatus for automatically adjusting the antenna direction to receive images with an optimum quality.
2. Description of the Prior Art
In order to automatically adjust the direction of an antenna for received images with an optimum quality in a system receiving broadcasting waves directly from a satellite, a method is known in the prior art by which the level of an automatic gain control (AGC) voltage for a received signal is measured and the antenna is steered so as to maximize the level of the AGC voltage. This type of method is disclosed, for instance, in Japanese Patent Laid-open No. 62-69737 entitled "Satellite Broadcasting Receiving System".
According to the conventional technique, the magnitude of an input signal is detected in terms of an AGC voltage and an antenna is steered in the direction where the maximum magnitude of an input signal is obtained. However, the AGC voltage is affected by the signal losses within cables interconnecting an outdoor unit and an indoor unit and the noises amplified excessively by an amplifier inserted to compensate losses. On the other hand, the quality of an image (S/N) is not necessarily proportional to the magnitude of the input signal but to the C/N (Carrier to Noise Ratio) of the input signal. That is, in the case of stronger noise components in the signal (a smaller C/N), the image quality becomes degraded even if the magnitude of the input signal is large, and on the contrary, in the case of weaker noise components in the signal (a larger C/N), the image quality becomes satisfactory even if the magnitude of the input signal is small.
Therefore, although the use of the AGC voltage for the detection of the input signal magnitude is effective, there is the possibility that images of an optimum quality may not be received by the automatic adjustment of an antenna direction using the AGC voltage since the image quality is not necessarily proportional to the magnitude of the input signal as described above.
Moreover, according to the conventional technique, when the antenna direction to receive the wave from another satellite adjoining an objective satellite exists in the range for the measurement of the AGC voltages and the signal received from the adjoining satellite is stronger than the one from the objective satellite, the antenna may be erroneously steered toward the adjoining satellite, since the direction of the antenna is controlled so as to maximize the level of the AGC voltage.